Technical Abstract:
Carbon and N contents in soils have long been recognized as important indicators of soil productivity. The current low levels of soil C and N of cropland soils have led to interest in requestering C with reduced tillage cropping systems and with the Conservation Reserve Program (CRP). Our objective was to determine the effect of agroecosystem on soil C and N pools in the Southern High Plains. These included three cotton (Gossypium hirsutum L.) cropping systems and two grassland systems (CRP or native rangeland (NR)). We sampled 0-5, 5-10, 10-20, and 20-30 cm soil depths at eleven farm sites in four counties. Each system had five or six replicates or farm sites. Total soil C and N, particulate organic matter (POM)-C and 'N, delta 13C of POM and whole soil, potentially mineralizable C and N, water-extractable-C (WEC), and extractable NH4 and NO3 were determined. Total C and N in the 0-30 cm soil profile was 33 Mg C/ha and 2.5 Mg N/ha for NR, and 22.4 Mg C/ha and 2 Mg N/ha for cropland systems, respectively. Total soil C and N in CRP were greater than in cropland soils only in the 0-5 cm surface layer. Labile C and N pools were positively correlated with each other and with total soil C and N. The relatively short time the CRP sites were in place (10-15 yr) apparently limited C and N sequestration.